1. Field of the Invention
The present invention relates to a journal bearing apparatus, and more particularly to a journal bearing apparatus capable of varying the clearance between a rotation shaft and a bush of a sleeve facing each other, or the clearance between an end of the rotation shaft and the surface of a thrust bearing facing each other.
2. Description of the Related Art
Recently, as computer-related industries have been developed, driving motors for various kinds of facilities, such as a head driving apparatus of a video tape recorder, an optical polygon driving apparatus of a laser printer, or a camcorder driving motor, require a high density and a miniaturized form. These driving apparatuses require a bearing which is precise and stable, and has a superhigh rotation performance. In compliance with this requirement, a journal bearing is generally used. In the journal bearings, to minimize friction which prevents the rotation of a rotation shaft, the vibration causing a low performance of the rotation shaft, and the noise of the rotation shaft, dynamic pressure generating grooves having several shapes have been developed.
Referring to FIG. 1, a scanner motor, i.e., an optical polygon driving device of a laser printer employing a conventional journal bearing, is illustrated. As shown in the drawing, a plate 60 having a rotor (not illustrated) is indentedly fixed at a hub 70, and an optical polygon 80 is mounted on the hub 70. The hub 70 is indentedly fixed at the rotation shaft 30. A sleeve 20 having a through hole therein is inserted into one side of a bearing bracket 10, fixed by screws. At the other side of the bearing bracket 10, a thrust bearing 50 is inserted and fixed to the sleeve 20 by screws.
The end of the rotation shaft 30 is inserted into the sleeve 20, and faces the thrust bearing 50. Moreover, a first dynamic pressure generating groove 35 is formed in the outer surface of the rotation shaft 30, and a bush 25 which protrudes from the inner surface of the sleeve 20 facing the first dynamic pressure generating groove 35, to support a radial load of the rotation shaft 30. On the surface of the thrust bearing 50, a second dynamic pressure generating groove 55 is formed to support a thrust load of the rotation shaft 30. Reference numeral 20a illustrates a vent hole.
The fluid pressure generated between the bush 25 and the outer surface of the rotation shaft 30 is illustrated as follows. The fluid pressure P generally formed between the bush 25 and the outer surface of the rotation shaft 30 is obtained by the following formula: EQU P=F/S
where, F is a weight of the rotation shaft and axial load, and S is a clearance area between the bush 25 and the rotation shaft 30. Here, assuming that F is constant, the fluid pressure P is in reverse proportion to the clearance area S.
As the clearance S between the bush 25 and the rotation shaft 30 is constant in the longitudinal direction of the first dynamic pressure generating groove 35, the fluid pressure P is always constant with respect to one side of the rotation shaft 30. However, as shown in FIG. 1, with respect to both sides, as the clearance S between the bush 25 and the rotation shaft 30 is not constant (that is, .DELTA.L1.noteq..DELTA.L2), the clearance area varies, and thereby a large fluid pressure is generated in the area where the clearance is narrow. To the contrary, where the clearance between the bush 25 and the rotation shaft 30 is wide, the fluid pressure is feeble.
Next, the fluid pressure generated between the end of the rotation shaft 30 and the thrust bearing 50 is explained as follows.
After the fluid flows into edge parts A and C of the second dynamic pressure generating groove 55 of the thrust bearing 50 by the rotation of the rotation shaft 30, the fluid turns and flows into a central part B of the second dynamic pressure generating groove 55, and thereby the fluid pressure P for raising the rotation shaft 30 is generated in the central part of the second dynamic pressure generating groove 55. As there is no variation in clearance area S between the thrust bearing 50 and the end of the rotation shaft 30, and in the weight of the rotation shaft and the axial load F, the fluid pressure P increases in proportion to the angular velocity. When the rotation shaft arrives at a predetermined number of rotations per minute, and the fluid pressure P is greater than weight of the rotation shaft and the axial load F, the rotation shaft 30 forms a predetermined clearance apart from the thrust bearing 50 and it is raised upwardly, thereby realizing an equilibrium state.
The time for the rotation shaft 30 to be raised is determined by both the clearance area between the thrust bearing 50 and the rotation shaft 30, and the rotational speed of the rotation shaft 30.
However, the conventional journal bearing apparatus has some problems as follows.
First, the stability of the rotation shaft is greatly declined due to the pressure variation generated between the bush and the rotation shaft. In other words, fluids are gathered at the folding part of the first dynamic pressure generating groove having a herringbone shape, generating a high fluid pressure. Due to the instability of the rotation shaft due to the concentration of the fluid pressure from the initial driving of the rotation shaft until the state of equilibrium, excessive vibration of the shaft is generated.
Furthermore, in the case that the rotation shaft stops, the center of the rotation shaft is eccentric for the center of the second dynamic pressure generating groove. When the rotation is performed under this state, as the center of the second dynamic pressure generating groove having the greatest fluid pressure and the center of the rotation shaft to be raised are incompatible with each other, the power is unbalanced and the rotation becomes unstable. Moreover, it takes a considerable period of time to square the center of the rotation shaft with the central part of the second dynamic pressure generating groove by the fluid pressure generated by the second dynamic pressure generating groove, thereby lower quality products are produced.